In a traveling type working machine such as a wheel loader or the like which is representative of a load working vehicle, a hydraulic pump and a torque-converter are driven by an engine to thereby drive respective working machines and track devices.
The engine is cooled by circulating a coolant (engine cooling water) to the main body of the engine. The coolant heated in the engine is passed through a radiator to be cooled, and then returned to the engine. Furthermore, the hydraulic pump and the torque-converter require hydraulic operating fluid. The hydraulic operating fluid is cooled by leading the hydraulic operating fluid to respective oil coolers.
The radiator and the oil cooler are cooled by air produced by a cooling fan. In general, the cooling fan is secured to an engine driving shaft and directly rotated by the engine. Furthermore, in consideration of a layout problem or a noise problem, a method of driving the cooling fan while the cooling fan is separated from the engine has been adopted.
For example, in JP, A 2000-303837, the cooling fan is driven by a hydraulic motor. In this case, the hydraulic motor is driven by the hydraulic fluid delivered by a hydraulic pump, and the hydraulic pump is driven by an engine. Furthermore, in JP, A 2000-303837, a coolant temperature and a hydraulic operating fluid temperature are detected, and the rotational speed of the cooling fan is controlled to the optimum rotational speed in accordance with these temperatures, whereby the cooling fan is driven at the optimum energy efficiency and the noise is controlled to the minimum level. The hydraulic pump is a variable displacement hydraulic pump, and the tilting angle of the hydraulic pump is controlled to vary the displacement volume (capacity) of the hydraulic pump, whereby the delivery capacity of the hydraulic pump is varied to control the rotational speeds of the hydraulic motor and the cooling fan.
Patent Document 1: JP, A 2000-303837